Abstract
Purpose :
Unilateral SO palsy is typically diagnosed by the 3-step test criteria requiring ipsilateral hypertropia, that increases in contralateral gaze, and with ipsilateral head tilt. I asked if this pattern, in congenital & acquired cases, is specific for SO atrophy that occurs reliably after SO denervation.
Methods :
In a prospective study of head-position dependent hypertropia, quasi-coronal, surface coil MRI was performed in central gaze, and in some cases supraduction, and infraduction. The hypertropic SO of 57 subjects had max. cross section <80% of contralateral, indicating unilateral SOP: 22 subjects had congenital (mean age 29±19 yrs, standard deviation, SD) & 35 had acquired strabismus onset (age 46±19 yrs, symptoms for 6±7 yrs). There were 26 similarly hypertropic subjects with normal SO muscles that were considered masquerading cases: 8 subjects had congenital (mean age 36±16 yrs) & 18 had acquired onset (age 34±18 yrs, symptoms for 6±9 yrs). Alignment was measured using prisms in diagnostic positions with & without head tilt, & with double Maddox rods.
Results :
Maximum SO cross section averaged 7.7±3.8mm2 in congenital & 10.7±3.5mm2 in acquired SOP (P=0.003), significantly less than contralesionally at 17.9±3.7 mm2 & 18.9±3.9mm2, respectively (P<10-9). Maximum hypertropic SO cross section in congenital (18.4±3.0 mm2) & acquired (20.7±3.1mm2) masquerading cases was statistically identical to contralateral in all groups (P>0.4). The contractile increase in SO cross section from up to down gaze was statistically similar in hypertropic & fellow eyes of masquerades (P>0.6). The 3-step test was fulfilled in all congenital but only 72% of acquired SO atrophy, tending even less than 81% in masquerades (P=0.27). Alignment & torsion measures were statistically indistinguishable in all diagnostic positions between cases of SO atrophy & masquerades, except that hypertropia in infraversion was slightly larger at 14.5±11.5Δ in SO palsy than 9.2±9.1Δ in masquerades (P=0.04). There were supranormal vertical fusional amplitudes of up to 27Δ in 56% of masquerading cases.
Conclusions :
Masquerading SO palsy occurs in the presence of normal SO morphology and contractile function, quantitatively mimicking all alignment features of actual SO unilateral weakness manifested by SO atrophy, including supranormal vertical fusional amplitudes with both congenital & acquired onset.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.